Piezoelectric thin-film acoustic wave device and information processing unit using the same

ABSTRACT

A piezoelectric thin-film acoustic wave device formed of a piezoelectric thin film of AlN on the +C plane and having the polarization strength of not lower than 0.63×10 −20  F/V and an information processing unit using the same are disclosed. This is the result of the inventors having studied the factors other than the C-axis orientation affecting the electromechanical coupling factor and developing a method of improving the electromechanical coupling factor in view of the occasional fact that the electromechanical coupling factor cannot be improved by improving the C-axis orientation and the electromechanical coupling factor required for the piezoelectric thin-film acoustic wave device is not obtained. In such a case, the receiving sensitivity of the receiving system may be deteriorated and the transmission strength of the transmission system is required to be increased undesirably having an adverse effect on the power saving efforts.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2005-158300 filed on May 31, 2005, the content of which is herebyincorporated by reference into this application.

BACKGROUND

This invention relates to a piezoelectric thin-film acoustic wave deviceand an information processing unit using the same.

In the conventional piezoelectric thin-film acoustic wave device asdescribed in “IEEE Transactions on Ultrasonics, Ferroelectrics, andFrequency Control, Vol. 47, No. 1, January p. 292 (2000)”, the loss ofthe piezoelectric thin-film acoustic wave device is considered to beeffectively reduced with a large electromechanical coupling factor and awide band, as a good orientation of the C axis of the crystal of thepiezoelectric thin film is satisfactory.

SUMMARY OF THE INVENTION

A detailed study of the present inventors, however, shows that theelectromechanical coupling factor may happen to be small in spite of asatisfactory orientation of C axis. Specifically, it sometimes happensthat in spite of an improved orientation of C axis, theelectromechanical coupling factor fails to be improved up to a levelrequired as a piezoelectric thin-film acoustic wave device. In such acase, the loss of the piezoelectric thin-film acoustic wave deviceincreases to such an extent that in an assumed application to a mobilecommunication unit, for example, the receiving sensitivity of thereceiving system may be deteriorated and the transmission strength ofthe transmission system is required to be improved undesirably forsaving power. In view of this, the present inventors have studied thefactors affecting the electromechanical coupling factor other than theC-axis orientation and a method of improving the electromechanicalcoupling factor.

Accordingly, it is an object of this invention to provide a reliablepiezoelectric thin-film acoustic wave device and an informationprocessing unit using the same.

In order to solve the problem described above, according to thisinvention, there is provided a configuration described in the claimsappended hereto.

According to this invention, it is possible to provide a reliablepiezoelectric thin-film acoustic wave device and an informationprocessing unit using the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a MOCVD apparatus.

FIG. 2 is a diagram showing typical conditions and procedure for forminga film.

FIG. 3 shows a SNDM image of an AlN film under the conditions shown inFIG. 2.

FIG. 4 is a diagram showing the film forming conditions 2 and 4 in FIG.10.

FIG. 5 shows a SNDM image of an AlN film under the film formingconditions in FIG. 10.

FIG. 6 is a diagram showing the relation between the electromechanicalcoupling factor and the polarization strength as observed under SNDM.

FIG. 7 is a graph showing the electromechanical coupling factor and thepolarization strength.

FIG. 8 is a graph showing the growth temperature and the polarizationstrength.

FIG. 9 is a diagram showing an application of the acoustic wave deviceaccording to this embodiment.

FIG. 10 is a diagram showing the AlN film forming conditions.

FIG. 11 is a diagram showing an AlN crystal structure.

FIG. 12 is a block diagram showing a mobile communication unit.

FIG. 13 is a diagram showing a configuration of a Duplexer.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have studied the factors other than the C-axisorientation affecting the electromechanical coupling factor and a methodof improving the electromechanical coupling factor.

First, the inventors focused on the polarization strength as a factoraffecting the electromechanical coupling factor other than the C-axisorientation. Specifically, the inventors conducted a verification testassuming that the polarization strength is a factor affecting theelectromechanical coupling factor and studied the possibility ofimproving the electromechanical coupling factor by controlling thepolarization strength. As a result, as shown in FIG. 7 (as described indetail later), it was made clear that the absolute value of thepolarization strength and the electromechanical coupling factor arecorrelated to each other, and that the larger the polarization strength,the larger the electromechanical coupling factor. It was also discoveredthat the absolute value of the polarization strength is correlated withthe growth temperature for the film forming process as shown in FIG. 8(as described in detail later). Specifically, it became apparent thatthe absolute value of the polarization strength and hence theelectromechanical coupling factor can be increased by controlling thegrowth temperature for the film deposition process.

The inventors also studied a method of improving the crystallinity ofthe piezoelectric thin-film acoustic wave device. A low crystallinitygives rise to the problem of displacement or dislocation of the crystalorientation, resulting in a smaller electromechanical coupling factor.As a result of the study by the inventors, it was discovered the degreeof crystallinity depends on the plane where crystal is grown.Specifically, assuming an AlN crystal structure shown in FIG. 11, thecrystallinity is varied between the +C plane growth and −C plane growth.Specifically, a more satisfactory crystal can be obtained in the case of+C plane growth than in the case of −C plane growth. A further studymade clear that the polarity, i.e. +C plane growth or −C plane growth,depends on the initial nitriding for the film forming process.Specifically, the polarity, i.e. the +C plane growth or the −C planegrowth can be controlled and the crystallinity can be improved for alarger electromechanical coupling factor depending on whether theinitial nitriding is carried out or not. A detailed explanation is madebelow with reference to the drawings.

First, the specifics of an experiment conducted to clarify the factorscontrolling the polarization strength and the polarity are explained.FIG. 1 is a schematic diagram showing an apparatus using MOCVD (metalorganic chemical vapor deposition) to form the AlN film. Although amethod is available in which ZnO or GaN is used instead of AlN, themethod using AlN is explained in the case under consideration. In FIG.1, ammonium gas (NH₃) or trimethyl aluminum (TMA) is used as a mainmaterial gas for the film forming process. Other conceivable materialsinclude aluminum (Al) target, nitrogen gas (N₂) and AlN powder.

Reference numeral 1 designates a cylinder filled with hydrogen, andnumeral 2 a cylinder filled with NH₃. Numeral 3 designates a refiningunit for producing a high-purity hydrogen gas and is arrangedimmediately after the cylinders. Numeral 4 designates a mass flowcontroller for controlling the flow rate of each material. Numeral 5designates a vertical reaction tube of quartz in which the film isformed by the reaction of the material gas. Numeral 6 designates ahigh-frequency induction heater for heating the substrate. Otherconceivable heating methods including the electro resistance heater.Numeral 7 designates a unit for bubbling TMA as one of the materialgases with hydrogen gas to produce a gas phase while at the same timebacking up the TMA gas with hydrogen gas and transporting it to areaction tube. To avoid reaction in the piping, NH₃ and TMA aretransported through different pipes and mixed in the reaction tube.

The inventors, paying attention to the conditions for AlN growthtemperature and NH₃ annealing (initial nitriding) as factors controllingthe polarization strength and the polarity, conducted the verificationunder various film forming conditions using the test equipment shown inFIG. 1. The initial nitriding is defined as a state in which the surfaceof a substrate (thin film formed in the initial stage) is nitridedbefore growing the AlN thin film.

FIG. 10 shows the conditions for forming an AlN film. The film formingcondition 1, or film deposition condition, represents the case free ofinitial nitriding. In this case, TMA is introduced after hydrogenannealing, and NH₃ is introduced 10 seconds later. The film formingcondition 2 concerns the case in which the NH₃ annealing process iscarried at the substrate temperature of 600° C. and the AlN film isformed at 600° C. A higher temperature may be employed. The film formingcondition 3 represents the case in which the NH₃ annealing is conductedat 1200° C. and the AlN film is formed at 600° C. The film depositioncondition 4 represents the case in which the NH₃ annealing process iscarried out under the same condition as the film deposition condition 3,i.e. at the temperature of 1200° C., and the AlN film is formed at 1200°C. Instead of the temperatures of 600° C. and 1200° C. employed as anexample of the image forming conditions in the aforementioned cases, thetemperatures of not lower than 600° C. and not lower than 1200° C.,respectively, may alternatively be employed.

FIG. 4 shows the experiments conducted under the film forming conditions2 and 4. The abscissa represents the time elapsed, and the ordinate thesubstrate temperature. In all film conditions, the substrate is heatedby increasing the temperature as quickly as possible to accuratelycontrol the hydrogen annealing process. After deposition of the film, onthe other hand, the temperature is decreased slowly over a long time tosuppress the residual distortion as far as possible.

The result of measurement by the scanning nonlinear dielectricmicroscope (SNDM) under the film forming conditions 1 to 4 shown in FIG.10 is illustrated in FIG. 5. In FIG. 5, the low color density indicatesthe −C plane growth, and the high color density the +C plane growth. Theaverage value Av of the polarization strength is the result ofmeasurement by SNDM, and the larger the absolute value of the averagevalue Av, the larger the polarization strength.

The polarization strength in terms of m/V and F/V is described below.

The term m/V is the unit of an output signal (Vout) of a lock-inamplifier directly obtained by measurement in the SNDM measurementsystem. This signal is varied depending on the measurement conditionseven with the same sample and not widely used. In other words, the samevalue can be obtained even for different samples by adjusting themeasurement conditions. The relative evaluation is possible with thisoutput signal for the same sample under the same measurement conditions.Under different conditions for the same sample, however, the evaluationshould be based on the output taking the measurement conditions intoconsideration. The signal strength (Ig) taking the measurementparameters into consideration is given by Equation 1 below.

$\begin{matrix}{{Ig} = {\frac{\frac{V_{out}}{10} \times \frac{S_{v}}{1000} \times \frac{\sqrt{2}}{4} \times R_{g}}{\frac{V_{a}}{2}} \times {1000\mspace{14mu}\left\lbrack {{Hz}\text{/}V} \right\rbrack}}} & (1)\end{matrix}$where Sv is the sensitivity in mV of the lock-in amplifier, Rg the rangein kHz of the FM demodulator, and Va the voltage in V applied to thesample. This signal strength of course reflects the nonlinear dielectricconstant. The evaluation more widely used with different materials is byderiving the tertiary nonlinear dielectric constant (∈^(ui)(3)) [F/V].This evaluation using a reference sample (with a known nonlineardielectric constant) is expressed as follows.

$\begin{matrix}{{ɛ^{ui}(3)} = {{ɛ^{st}(3)} \times \frac{S_{nl}\left( {ɛ^{st}(2)} \right)}{S_{nl}\left( {ɛ^{ni}(2)} \right)} \times \frac{\Delta\; f_{d}^{ui}}{\Delta\; f_{d}^{st}} \times {\left( \frac{f_{s}^{st}}{f_{s}^{ui}} \right)^{2}\mspace{14mu}\left\lbrack {F\text{/}V} \right\rbrack}}} & (2)\end{matrix}$Δf _(d) I _(g) ×V _(out)  (3)

where affixes “st” and “ui” designate a reference sample and ameasurement sample, respectively, ∈^(st)(3) the nonlinear dielectricconstant [F/V] of the reference sample which is the negative plane ofZ-cut LiTaO₃ having the value of −2.26×10⁻¹⁹ in this case, and Sn1(∈(2))the capacitance change susceptibility per unit nonlinear dielectricconstant which is the function of only the relative dielectric constant∈(2). The capacitance change susceptibility (Sn1(∈^(st)(2)) of thereference sample is 0.17075, and the capacitance susceptibility(Sn1(∈^(ui)(2)) of the measurement sample is 0.199927 which is a valuecalculated using the relative dielectric constant of sapphire. Also,“fs” is the resonance frequency changed by the capacitance generated inthe neighborhood of the sample surface and a probe when the probe isbrought into contact with the sample. The polarization strengthexpressed in terms of “%” is explained below.

This expression is defined as a polarizability in the sample plane. Theelectromechanical coupling factor 0.25% of the C-plane AlN film isassumed to be the in-plane polarizability of 100% (FIG. 7). In view ofthese facts, the result of an experiment under each film formingcondition is explained below. Specifically, with regard to the filmforming condition 1, the the AlN film formed is found to have grown onthe −C plane with the polarization strength of 2628 mV (or −1.00×10⁻²⁰F/V). Also, with regard to the film forming condition 2, the −C planegrowth is involved with the polarization strength of 2381 mV (or−1.51×10⁻²⁰ F/V). In other words, a substantially similar result isobtained with the −C plane growth in the case where the NH₃ annealing isnot conducted and in the case where the NH₃ annealing is conducted at600° C. With regard to the film forming condition 3, it is found thatthe +C plane growth is involved. Comparison with the film formingcondition 2 shows that the change occurs from −C to +C plane growth bythe NH₃ annealing at 1200° C. Further, with regard to the film formingcondition 4 at the growth temperature of 1200° C., the +C plane growthis involved with a higher polarization strength.

From the facts described above, it has become apparent that thepolarization strength can be increased by a higher growth temperature.Also, the AlN film having a high crystallinity with the −C plane growthhas yet to be reported, and the +C plane growth improves thecharacteristics of a device in applications. It is considered desirable,therefore, to increase the polarization strength by causing the +C planegrowth by the initial nitriding while at the same time increasing thegrowth temperature.

Next, the result of a verification test conducted based on the foregoingstudy is explained. FIG. 2 shows an example of typical film formingconditions and a typical film forming method according to thisembodiment. The substrate is formed of sapphire.

First, the annealing in a hydrogen atmosphere is carried out to cleanthe oxide layer and the damaged layer on the substrate surface. Thehydrogen annealing is conducted at the substrate temperature of 1200° C.for 15 minutes in a hydrogen atmosphere under the inner pressure of 30Torr of the reaction tube. After the hydrogen annealing, NH₃ isintroduced and the initial nitriding of the substrate surface isconducted for one minute. The substrate temperature is set to 1200° C.,the NH₃ flow rate to 300 sccm and the internal pressure of the reactiontube to 30 Torr. After the initial nitriding, TMA is introduced to forman AlN film. The film forming conditions are shown in FIG. 2.

FIG. 3 shows the polarity of the AlN formed under the conditions shownin FIG. 2 and the result of polarization strength measurement. As shownin FIG. 3, the AlN film formed has grown on C plane, of which theaverage value of the output signal (indicating the polarizationstrength) under SNDM with the +C plane growth is −2251 mV (or 1.43×10−20F/V or 83%). This is a comparatively large value in terms of an absolutevalue of the polarization strength of the AlN film, and therefore asufficient electromechanical coupling factor is obtained, while the +Cplane growth produces a satisfactory crystal.

The relation between polarization strength and electromechanicalcoupling factor K² and the relation between growth temperature andpolarity are described in detail with reference to FIGS. 6 to 8. Asshown in FIG. 10, AlN is polarized considerably in the direction of +Caxis or −C axis. The presence of C axis inverted in the plane generatesthe polarization in the opposite direction, resulting in thepiezoelectricity being offset. This is considered to deteriorate theelectromechanical coupling factor constituting the conversionefficiency. The measurement under SNDM was conducted, therefore, forpurposes of confirming the distribution of the polarity inversion andevaluating the relation between polarization strength andelectromechanical coupling factor. The measurement result is shown inFIG. 6. The measurement was using a sample (a) about 0.15% in K², asample (b) about 0.15% in K² (different from sample (a)) and a sample(c) about 0.1% in K². A similar polarization strength is exhibited bythe samples (a) and (b) having the same value K². The larger the valueK², the better, while the smaller the value K², the greater thedeterioration.

With regard to the sample (c) with a relatively deteriorated K², theSNDM image shows that the polarization strength is lower than those forsamples (a) and (b). The average value of the polarization strength is−831 mV indicating that the polarization strength and theelectromechanical coupling factor are correlated with each other. FIG. 7is a graph showing the correlation between polarization strength andelectromechanical coupling factor. The solid line represents anapproximation line based on the plot of actual measurement. The idealsaturation value of the electromechanical coupling factor for C-planeAlN is about 0.25%, and in order to achieve this value, the absolutevalue of the polarization strength is required to be controlled at 2000mV (or 1.27×10⁻²⁰ F/V or 80%) or higher. FIG. 8 is a graph showing thegrowth temperature and the polarization strength. It is understood thatin order to secure the polarization strength of at least 2000 mV forobtaining the ideal electromechanical coupling factor, the growthtemperature of not lower than 1100° C. is required for the +C planegrowth and not lower than 600° C. for the −C plane growth.

As understood from the foregoing description, the polarization strengthand the electromechanical coupling factor are correlated to each otherand the polarization strength is required to be controlled at 2000 mV(or 1.27×10⁻²⁰ F/V or 80%) or higher to obtain the idealelectromechanical coupling factor. Also, the +C plane growth requiresthe growth temperature of not lower than 1100° C. so that thepolarization strength can be controlled at 2000 mV (or 1.27×10⁻²⁰ F/V or80%).

The electromechanical coupling factor, on the other hand, is desirablynot less than 0.1% (not smaller than about 1000 mV (or 0.63×10⁻²⁰ F/V)in terms of the absolute value of polarization strength) taking thedevice characteristics into consideration. As far as this value issatisfied, the required characteristics of the filter of the oscillatorused with, for example, a mobile communication terminal can be obtained.Further, a value of not less than 0.2% substantially doubles the rangeof the application band as compared with the case of 0.1%. Furthermore,a value of not less than 0.5% or 1.0% makes possible a still widerapplication to the intermediate frequency filter of a mobilecommunication terminal.

Although the proportional relation between the maximum value 3000 mV (or1.89×10⁻²⁰ F/V) of the absolute value of polarization strength and themaximum value 0.25% of electromechanical coupling factor is plotted as agraph in FIG. 7, a similar proportional relation can be sustained forlarger values. The electromechanical coupling factor of 1.0% for 10000mV (or 6.3×10⁻²⁰ F/V) in the absolute value of the polarization strengthand the electromechanical coupling factor of 2.0% for 20000 mV (or1.26×10⁻¹⁹ F/V) in the absolute value of the polarization strength aresome examples. The electromechanical coupling factor, however, islimited to about 5.0% for AlN, about 3.0% for ZnO and about 1.0% forGaN, each of which is the upper limit for the corresponding material. Anapplication of the acoustic wave device using this piezoelectric thinfilm is shown in FIG. 9. An acoustic surface wave device and apiezoelectric thin film resonator are main examples of possibleapplication. The control technique and the applications described aboveare true also with other piezoelectric thin films of such materials asZnO and GaN as well as the AlN film.

FIG. 12 shows a configuration using the piezoelectric acoustic wavedevice for a mobile communication unit according to this embodiment.Numeral 101 designates an antenna for receiving a radio wave, numeral102 a Duplexer, numeral 103 a low-noise amplifier, numeral 104 a poweramplifier, numerals 105, 106 filters between receiving stage, numeral107 an RF IC, numeral 108 a baseband IC and numeral 109 an oscillatorfor driving the RF IC. The Duplexer 102 is shown in detail in FIG. 13.Numeral 111 designates a phase shifter, numeral 112 a receiving topfilter and numeral 113 a transmission top filter. The piezoelectricacoustic wave device according to this embodiment is used for thereceiving top filter 112, the transmission top filter 113 and also thefilter of the oscillator 109. As described above, the use of thepiezoelectric acoustic wave device according to this embodiment providesa reliable mobile communication unit.

This invention is described above with reference to embodiments, towhich the invention is not limited. It is apparent to those skilled inthe art that this invention can be altered or modified variously withoutdeparting from the spirit and the scope of the appended claim.

1. A piezoelectric thin-film acoustic wave device wherein: thepolarization strength of a piezoelectric thin film is not lower than0.63×10⁻²⁰ F/V; and the piezoelectric thin film is formed of selectedone of AlN, ZnO and GaN.
 2. A piezoelectric thin-film acoustic wavedevice according to claim 1, wherein the piezoelectric thin film isformed on the +C plane of crystal of the piezoelectric thin film.
 3. Aninformation processing unit comprising the piezoelectric thin-filmacoustic wave device according to claim
 1. 4. A piezoelectric thin-filmacoustic wave device wherein: the polarization strength of apiezoelectric thin film is not lower than 0.63×10⁻²⁰ F/V; and thepiezoelectric thin film is formed on the +C plane of the crystal of thepiezoelectric thin film.
 5. An information processing unit comprisingthe piezoelectric thin-film acoustic wave device according to claim 4.6. A piezoelectric thin-film acoustic wave device comprising apiezoelectric thin film of AlN, wherein the polarization strength of thepiezoelectric thin film is not lower than 1.26×10⁻²⁰ F/V and formed ofAlN on the +C plane of crystal of the piezoelectric thin film.
 7. Aninformation processing unit comprising the piezoelectric thin-filmacoustic wave device according to claim
 6. 8. A piezoelectric thin-filmacoustic wave device comprising: a piezoelectric thin film of AlN;wherein the polarization strength of the piezoelectric thin film is notlower than 1.26×10⁻²⁰ F/V and formed of AlN on the +C plane of crystalof the piezoelectric thin film; and wherein the piezoelectric thin filmis nitrided at the temperature of not lower than 1200° C. andsubsequently formed at the temperature of not lower than 600° C.
 9. Aninformation processing unit comprising the piezoelectric thin-filmacoustic wave device according to claim
 8. 10. A piezoelectric thin-filmacoustic wave device wherein: the polarization strength of apiezoelectric thin film is not lower than 0.63×10⁻²⁰ F/V; thepiezoelectric thin film is formed at the temperature not lower than 600°C.; and the piezoelectric thin film is formed on the +C plane of crystalof the piezoelectric thin film.
 11. An acoustic surface wave devicecomprising: both a piezoelectric thin film and an electrode respectivelyprovided on a substrate; wherein a polarization strength of apiezoelectric thin film is larger than 0.63×10⁻²⁰ F/V, and wherein thepiezoelectric thin film is formed of a selected one of AlN, ZnO and GaN.12. A piezoelectric thin-film resonator comprising: both a piezoelectricthin-film and an electrode respectively provided on a substrate; whereina polarization strength of a piezoelectric thin film is larger than0.63×10⁻²⁰ F/V, and wherein the piezoelectric thin film is formed of aselected on of AlN, ZnO and GaN.